This invention relates to a temperature measuring circuit equipped with a variable resistance-type temperature sensor such as a thermistor for heating and cooking appliances.
In general, in heating and cooking appliances such as bread baking devices and electronic ranges, a temperature sensor such as a thermistor for temperature measuring at a heating place is installed. The said temperature of the heating place is measured by the change of electrical resistance of said temperature sensor.
up to the present, data of potential differences between terminals of said temperature sensor installed in any place where the temperature is to be measured are memorized in a computer. By comparing said data with data of potential difference corresponding to the specified temperature memorized beforehand in said computer, the temperature in said place can be established. (Usually in this case, said prior memorized data are required for many hundred different temperatures.)
In general, the above-mentioned measuring system requires substantial memory for storing such amounts of data, which increases the assembly cost. Accordingly, for home appliances requiring temperature detection at only a few points (for example, 23.degree. C., 28.degree. C. and 32.degree. C. are required for bread baking devices), a simplified measuring circuit, as described in the following, may be used.
The simplified temperature measuring circuit in accordance with the invention is composed of a plurality of fixed resistances operating in the same voltage as the voltage between terminals of said temperature sensor corresponding to the specified individual temperature to be detected, and said fixed resistances are exchangeably connected from time to time according to the required heating procedures, and by comparing said voltage generated in terminals of said sensor with the specified secondary voltage (usually equal to a half of the first standard voltage), it is checked whether the present temperature is the specified temperature or not.
On the other hand, in said temperature measuring circuit, as said temperature sensor such as the thermistor is exposed to high temperatures, there is a danger of breaking said sensor, and disconnecting a connector or having it fall out. When these deficiencies occur, even if said temperature to be detected has already reached the specified temperature, it is possible to cause overheating because of mis-detecting of that said temperature will be under said specified value owing to said detecting element kept in high resistance.
Although said sensor breakage is detectable by analyzing said data taken in said computer in the aforementioned system taking said data of potential difference between terminals of said temperature sensor in said CPU through an A/D convertor, it is very hard to detect said deficiency in this simplified temperature measuring system.